A phosphor sandwich (phoswich) detector is a type of radiation detector commonly used for detecting multiple types of radiation in a mixed radiation field. A phoswich detector has a combination of scintillators optically coupled to a single photodetector, typically a photomultiplier tube. The scintillators are selected so that they are sensitive to different radiation types and have different decay times. Consequently, a particular type of radiation incident on the device will most likely interact with the corresponding scintillation layer and produce a photomultiplier pulse with a characteristic shape. Analysis of the shape of the output pulse from the photomultiplier tube can then be used to determine the type of incident radiation.
For example, U.S. Pat. No. 5,317,158 teaches an integrated alpha-beta-gamma scintillation detector, where the scintillators have three different decay constants and are separately sensitive to alpha, beta, and gamma radiation. Pulse analysis identifies pulse time constants to discriminate between alpha, beta, and gamma events. WO 2007005442 teaches a technique for digital pulse shape analysis for beta-gamma coincidence detection. The detector and pulse shape analysis (PSA) technique identifies detection in one or the other scintillator, and a superposition pulse shape for coincident events in both scintillators. U.S. Pat. No. 7,342,231 describes a two-layer system for beta and gamma coincidence measurements.
A recognized problem with existing phoswich detectors arises due to interaction of scintillators with undesired radiation types. For example, scintillators selected for beta sensitivity can nonetheless still interact with gamma-rays, and gamma scintillators can interact with beta-rays. If not properly taken into account and compensated for, these cross-talk effects result in inaccurate classification of radiation events. Thus, a beta detector must employ some technique to isolate beta events in a mixed beta-gamma radiation field.
For example, U.S. Pat. No. 5,008,546 discloses a technique for detecting beta radiation in the presence of background gamma radiation. The detector actually consists of two separate phoswich detectors positioned in close proximity. The first detector has a scintillator sensitive to both beta and gamma radiation, and a second scintillator is sensitive only the gamma radiation, e.g., using the same scintillator material together with a filter to block beta radiation. The two signal rates from two photomultiplier tubes are subtracted to derive the beta signal rate. Unfortunately, this approach requires two separate detectors and is subject to inaccuracies due to differences between the detectors. Moreover, this detector is not able to provide spectroscopic information from either one of the radiation types.
A similar technique uses just one detector but makes two measurements separated by time. The first “open window” measurement is sensitive to both beta particles and gamma rays, and thus includes the undesired gamma ray events together with the desired beta events. The second measurement is made with the filter in place. The incident beta-particle energy distribution is then calculated by subtracting the second measured distribution from the first. Unfortunately, this technique requires making two measurements separated in time, and it also assumes that the ambient gamma-ray field and device characteristics remain constant during the time interval between the two measurements.
Another approach to compensate for cross-talk effects in phoswich detectors is to estimate a constant fraction of mischaracterized gamma events and subtract that fraction from the measured beta events. Unfortunately, this approach assumes the fraction to be constant and/or requires recalibration of the estimated constant depending on the specifics of the mixed radiation field. Moreover, it does not allow calculation of the beta energy absorption.
In summary, current phoswich detectors use one scintillator layer for each distinct type of radiation to be detected. Existing techniques to correct for cross-talk between these layers in mixed radiation fields are not completely satisfactory. Accordingly, there remains a need for improved phoswich detectors that can more accurately and reliably discriminate between distinct radiation types.